As a button that is commonly used for jeans, jean jacket, etc., a type of a button is known, which has a generally cylindrical barrel and a head expanding in a disk form at one end of the barrel, wherein the head is taken in and out through a buttonhole. Such a button is disclosed in, for example, US Patent Application Publication No. 2007/0226959 (Patent Document 1). This type of button comprises three parts, namely, a button back and a shell cap, which are metallic outer shell parts and an insert, which is a resin or metallic core part. The button back comprises a cylindrical part defining a periphery wall of the barrel of the button and an umbrella part extending radially outward from one end of the cylindrical part, defining a back side wall of the head of the button. Generally, a radially outward end portion of the umbrella part is pressed so as to bend on the opposite side of the cylindrical part. Thereby, the umbrella part is given a thickness for a part of the insert corresponding to the head to be placed, and the strength of the umbrella part is increased. The shell cap comprises a disk part defining a front side wall of the head of the button and a bending part which is bent from the radially outward end of the disk part on the back side of the head. The bending part is deformed to be attached a curved, radially outward end portion of the umbrella part of the button back. The front surface of the disk part of the shell cap becomes the front surface of the head of the button, and generally onto this surface, logos or designs are added, which comprise characters, figures, symbols or the like with orientations thereof in the circumferential direction have been set. Such a button is mounted to jeans or the like with the predetermined orientation.
If a shell cap with an oriented logo or the like on the surface as mentioned above is rotated, namely circumferentially displaced with respect to the umbrella part of a button back after the shell cap was attached to the umbrella part, then the logo or the like is displaced from the predetermined orientation. Further, even if a shell cap has no orientation, it is not preferable that the shell cap can be rotated in use from the aspect of product quality. To prevent such a shell cap from being rotated, the Patent Document 1 suggests that the outer periphery of an umbrella part of a button back is formed in a polygonal shape (see FIG. 9 of the document) in order to make the corners of the polygonal shape work as resistances to rotating of the shell cap. In this case, however, there is a problem that it is hard to form the front surface of the button to be a beautiful circle since the bending part of the shell cap is deformed along the polygonal shape of the umbrella part.
As another button back having a function of preventing the shell cap from rotating, one is known in which a plurality of bumps are circumferentially provided on a curved, radially outward end portion of the umbrella part as resistances to rotating of the shell cap. However, to form such bumps, a die becomes complicated, increasing production costs. Further, to reduce material costs of the button, button backs have been produced using aluminum or aluminum alloy instead of conventional brass, recently. In a case of a button back made of aluminum or aluminum alloy, even if the above-mentioned bumps are provided on the radially outward end portion of the umbrella part, the shell cap may rotate for the following reason. That is, the radially outward end portion itself of the umbrella part is easily deformed with the shell cap attached to the radially outward end portion, and when the umbrella part is deformed, the attachment force will be lowered, resulting in rotating of the shell cap.